Re: Old row version in hot chain become visible after a freeze

> > The
> > resulting patch, which I like better than the previously proposed idea
> > of skipping the freeze, takes the approach of handling freeze correctly
> > for the cases where the tuple still exists after pruning.
>
> That's also something I was wondering when looking at the first patch.
> I am unfortunately not as skilled as you are with this area of the
> code (this thread has brought its quantity of study!), so I was not
> able to draw a clear line with what needs to be done. But I am clearly
> +1 with this approach.

I have tried out the new patch and it works great as well! +1 from me.

> > I also tweaked lazy_record_dead_tuple to fail with ERROR if the tuple
> > cannot be recorded, as observed by Yi Wen. AFAICS that's not reachable
> > because of the way the array is allocated, so an elog(ERROR) is
> > sufficient.

I agree the fail is rare (and probably doesn't happen in real cases, although the comment
does imply with a sufficiently low working_memory it might?). However, I'd dispute that ERROR
is sufficient --PANIC is probably appropriate here because FREEZE is not WAL-logged until
the end of the page; so we'd end up with unfrozen Xids hanging around with an appropriately
timed crash. I wouldn't worry much about the PANIC tripping because like you said,
this seems unreachable normally.

The errmsg should come with a errhint saying "Increase maintenance_work_mem".

I've done some code inspection to try to figure out if crashing at any point around before FREEZE and
the later lazy_vacuum_page phase is safe (i.e. older Xids might be left behind on restart/crash); it seems to me it
shouldn't be a problem until new_min_xid and new_min_multi isn't updated until later for the truncate.

Yi Wen